I dont think this is directly moddable but a workaround is problerably the best option. Basically you have to trick the game into thinking that there isn't a unit there. This is generally how to bypass hardcoded stuff.
The most difficult part of this is that units are comparitively easy to mod compared to the other aspects of RTW but they have a very strict system as they are complicated for the engine to work with. You generally find this with computers - if there's a system that is easy for humans to understand its very hard for the computer to understand and so much less flexible on the computer's end.
On the plus side the fellbeasts dont look too bad in the youtube video. I suppose it depends how high you want them to fly - whether its just hovering or full on flying high in the sky.
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